1. Field of the Invention
This invention resides in the field of thermoelectric devices and the heat sinks used in conjunction with these devices.
2. Description of the Prior Art
Thermoelectric devices are widely used for heating metal blocks that hold reaction receptacles in chemical and biochemical laboratories, particularly multiple tubes or multi-receptacle plates. The metal blocks often referred to as “sample blocks,” and the typical sample block contains a planar array of depressions or wells with a separate sample receptacle in each well. Procedures that are commonly performed on samples in a sample block involve keeping each sample under close temperature control and heating and cooling the samples in discrete, programmed steps.
The polymerase chain reaction (PCR) is one of many examples of chemical processes that are performed on multiple samples and require precise temperature control with rapid temperature changes between different stages of the procedure. PCR amplifies DNA, i.e., it produces multiple copies of a DNA sequence from a single copy. PCR is typically performed in instruments that provide reagent transfer, temperature control, and optical detection in a multitude of reaction vessels such as microplates, tubes, or capillaries. The various stages of the procedure are temperature-sensitive, with different stages performed at different temperatures and maintained for designated periods of time, and the sequence is repeated in cycles. In a typical procedure, a sample is first heated to about 95° C. to “melt” (separate) double strands, then cooled to about 55° C. to anneal (hybridize) primers to the separated strands, and then reheated to about 72° C. to achieve primer extension through the use of the polymerase enzyme. This sequence is repeated to achieve multiples of the product DNA, and the time consumed by each cycle can vary from a fraction of a minute to two minutes, depending on the equipment, the scale of the reaction, and the degree of automation. Another example of a chemical process that involves temperature changes and a high degree of control is nucleic acid sequencing. Still further examples will be apparent to those knowledgeable in the fields of molecular biology and biochemistry in general.
The processes cited above are frequently performed on large numbers of samples, each of a relatively small volume, often on the microliter scale, using automated laboratory equipment. A central component of this equipment is the reaction module, which includes the sample block, a thermoelectric device or array of such devices contacting the underside of the sample block, and a heat sink associated with the thermoelectric device, all with appropriate thermal interfaces to achieve maximal heat conduction. One example of such a module is shown in Atwood, J. G., et al. U.S. Pat. No. 7,133,726 B1. The heat sink in Atwood et al. includes a “generally planar base 34” that contacts the thermoelectric devices directly and a series of fins 37 extending downward from the base. A “trench 44” is cut into the base 34 outside the perimeter of the thermoelectric device to limit heat conduction and to decrease edge losses from the area bounded by the trench (column 8, lines 9-13). The patent states that, heat loss at the corners of a rectangular sample block is greater than at other locations on the block, causing the corners to become cooler (column 5, lines 40-41). The patent recommends the placement of insulation around the corners to control this heat loss, and the use of a small thermal connection from the center of the sample block to the heat sink that acts as a “heat leak” to reduce the temperature in the center of the block and thereby maintain a more uniform temperature across the block (column 5, lines 44-54).